The present invention relates to a drive device using an electromechanical transducer, and to an apparatus employing a drive device using the electromechanical transducer such as a XY moving stage for precision measurement, a photo-taking lens for cameras, a projection lens for overhead projectors and a binoculars lens and the like.
When drive pulses each consisting of a gradual rising portion and a steep falling portion following it are applied on a piezoelectric transducer, the piezoelectric transducer is displaced to gradually expand in the thickness direction at the gradual rising portion of each of the drive pulses, and is displaced to rapidly contract at the steep falling portion. Thus, there has been known a drive device (as an example, refer to Japanese Patent Laid-Open Application No. 6-123830) which taking advantage of this characteristic property, applies such wavy drive pulses as described above on the piezoelectric transducer to repeat charging and discharging at different velocities, causes in the piezoelectric transducer vibrations having different velocities in the thickness direction to reciprocate a driving member fixed to the piezoelectric transducer at different velocities, and moves a moving member frictionally coupled with the driving member in a predetermined direction.
FIG. 18 is a sectional view showing an example of the constitution of a photo-taking lens driving device for cameras using the above-described drive device. In the figure, numeral 101 designates a lens barrel, to the left end of which a holding frame 102 of a first lens L1 is fixedly mounted, and the right end 101a of which forms a holding frame of a third lens L3. Inside the lens barrel 101, a holding frame 103 of a second lens L2 is movably disposed in a direction of the optical axis. Numeral 104 designates a drive shaft for driving the lens holding frame 103 in the direction of the optical axis, and the drive shaft 104 is movably supported by a first flange portion 101b of the lens barrel 101 and a flange portion 102b of the lens holding frame 102 in the direction of the optical axis, and one end thereof is fixedly adhered to one of surfaces of the piezoelectric transducer 105.
The piezoelectric transducer 105 is disposed in the thickness direction to dispose the drive shaft 104 in the axial direction, and one end surface of the piezoelectric transducer is fixedly adhered to the drive shaft 104, and the other end surface thereof is fixedly adhered to the second flange portion 101c of the lens barrel 101.
The lens holding frame 103 for holding the second lens L2 has a slider block 103b which is a moving member extending upwardly. The drive shaft 104 penetrates the slider block 103b in the horizontal direction. An opening portion 103c is formed above a portion of the slider block 103b where the drive shaft 104 penetrates whereby an upper half portion of the drive shaft 104 is exposed. A pad 106 which is brought into contact with an upper half portion of the drive shaft 104 is inserted into this opening portion 103c, a projection 106a is provided on the upper portion of the pad 106, the projection 106a of the pad 106 is pressed down by a flat spring 107, and a downward urging force F is applied on the pad 106 at a portion thereof that is brought into contact with the drive shaft 104. FIG. 19 is a sectional view showing a constitution of a frictionally-coupled portion between the drive shaft 104, and the slider block 103b and the pad 106.
Next, the description will be made of the control operation. When it is necessary to move the lens L2 in a direction indicated by an arrow a, such drive pulses each consisting of a gradual rising portion and a steep falling portion following it as shown in FIG. 20 are supplied to the piezoelectric transducer 105.
At the gradual rising portion of each of the drive pulses, the piezoelectric transducer 105 is disposed to gradually expand in the thickness direction, and the drive shaft 104 is displaced in a direction indicated by the arrow a in the axial direction. This causes the drive shaft 104 to be brought into press contact by a flat spring 107 to move the slider block 103b and the pad 106 which have been frictionally coupled in the direction indicated by an arrow a, and therefore, the lens holding frame 103 can be moved in the direction indicated by the arrow a.
At the steep falling portion of each of the drive pulses, the piezoelectric transducer 105 is rapidly disposed to contract in the thickness direction, and the drive shaft 104 is also displaced in a direction opposed to the direction indicated by the arrow a in the axial direction. At this time, the slider block 103b, the pad 106 and the lens holding frame 103 which have been brought into press contact with the drive shaft 104 by the flat spring 107 substantially stay at their positions by surpassing the frictional force between the slider block 103b, the pad 106 and the lens holding frame 103, and the drive shaft 104 by the inertia thereof, and the lens holding frame 103 is not moved.
Incidentally, according to the expression "substantially" mentioned here, there is included a movement of the slider block 103b where the slider block 103b follows the drive shaft 104 while slipping at frictionally coupled faces between the slider block 103b, the pad 106 and the drive shaft 104 in any of a direction indicated by an arrow a and the direction opposed thereto and the slider block 103b is moved in the direction indicated by an arrow a as a whole by a difference in drive time periods.
By continuously applying the above-described drive pulses on the piezoelectric transducer 105, it is possible to continuously move the lens holding frame 103 in a direction indicated by the arrow a.
A movement of the lens holding frame 103 in a direction opposed to the direction indicated by the arrow a can be achieved by applying wavy drive pulses each consisting of a steep rising portion and a gradual falling portion following it on the piezoelectric transducer 105.
In the above-described drive device using the piezoelectric transducer, there has been adopted a method of driving by applying saw tooth wave pulses generated by a drive pulse generating circuit on the piezoelectric transducer, or combining a constant-current charging circuit with a short-circuit discharging circuit to apply drive pulses each consisting of constant-current charging and rapid discharging, or applying drive pulses each consisting of rapid charging and constant-current discharging on the piezoelectric transducer.
On the other hand, in the drive device of this sort, precise positioning is generally achieved by making the driving velocity slow. In the drive device using the piezoelectric transducer, the driving velocity can be made slower by lowering the voltage of the drive pulses to reduce the expansion and contraction displacements thereof, but there is a drawback where the thrust force (driving force) also becomes smaller at the same time, and the driving velocity and the thrust force become unstable.
Particularly in the case of a fine moving distance, a voltage to be applied also becomes very low, the thrust force (driving force) also become small, and the moving distance and thrust force become unstable, resulting in a drawback where it becomes difficult to position precisely.
FIG. 1 is a diagram showing relationship between voltage of drive pulses to be applied on the piezoelectric transducer and driving velocity, showing that as the voltage of drive pulses increases, the driving velocity also becomes faster, and that as the voltage of the drive pulses lowers, the driving velocity also lowers, but that when the voltage of drive pulses lowers below a predetermined critical value p, the driving velocity becomes unstable.
FIG. 2 is a diagram showing relationship between voltage of drive pulses to be applied on the piezoelectric transducer and thrust force, showing that as the voltage of drive pulses increases, the thrust force also increases, and that as the voltage of the drive pulses lowers, the thrust force also decreases, but that when the voltage of drive pulses lowers below a predetermined critical value q, the thrust force becomes unstable.
Further, FIG. 3 is a diagram showing relationship between a frequency of drive pulses to be applied on the piezoelectric transducer and the driving velocity, showing that as the frequency of drive pulses increases, the driving velocity also becomes faster, and that as the frequency of the drive pulses lowers, the driving velocity also decreases.
In this case, the driving velocity does not become unstable at low frequencies, but there is a drawback where vibration sound generated is heard as noises to the human ear at an audio frequency or less (about 20 kHz or less).
FIG. 4 is a view showing a waveform of drive pulses when the drive pulses are thinned out every a specified period of time without changing the voltage and frequency of drive pulses to be applied on the piezoelectric transducer, and is advantageous in that the voltage and frequency are maintained, but the driving becomes intermittent because the drive pulses are thinned out. For this reason, there is a drawback where vibration sound corresponding to a period of the intermittent driving is generated and is heard as noises to the human ear.
As a countermeasure against this, it is considered to reduce a number of lamination layers of an unit element constituting the piezoelectric transducer without changing the voltage and frequency of drive pulses applied on the piezoelectric transducer in order to make the driving velocity slower. That is, if the unit element constituting the piezoelectric transducer is divided into a plurality of blocks and drive pulses of a sufficient amplitude are applied to only some of those blocks, the thrust force (driving force) remains unchanged and the driving velocity does not become unstable because the expansion and contraction displacements of the unit element do not change.
In order to conduct precise positioning in a drive device of this sort, there is a method for moving to a target position by switching to low-velocity driving which causes desired expansion displacement by applying DC voltage on the piezoelectric transducer after high-velocity driving which applies drive pulses on the piezoelectric transducer to cause reciprocal displacements and moves a driven member to near a desired position.
In such low-velocity driving, since a movable distance is in proportion to a number of lamination layers of an unit element constituting the piezoelectric transducer, it will suffice if the number of lamination layers of the unit element is increased to increase the moving distance by low-velocity driving. When, however, the number of lamination layers of the unit element is increased, the resonance frequency of a driving system including the piezoelectric transducer lowers. For this reason, when low-velocity driving is conducted using the piezoelectric transducer having a large number of lamination layers of the unit element, there are no particular problems, but when this piezoelectric transducer (piezoelectric transducer having a large number of lamination layers of the unit element) is commonly used also for high-velocity driving, there is a drawback where vibrations of audio frequency or less (20 kHz or less) are generated and the vibration sound is heard as noises to the human ear.
In case where the piezoelectric transducer is divided into a plurality of blocks, when design is made such that numbers of unit elements of piezoelectric transducers constituting the respective blocks are different in order to obtain a desired driving velocity, the impedance differs with the block. In other words, since the impedance of the piezoelectric transducer, which is a load as viewed from the drive pulse circuit side, changes in accordance with a block selected to obtain a desired driving velocity, there arises a case where drive pulses each having the desired waveform cannot be applied on the piezoelectric transducer. Namely, there arises a drawback where the desired driving velocity cannot be obtained simply depending upon a change in the number of lamination layers of the piezoelectric transducer.
Further, in a drive device of this sort, as described above, the driving velocity is generally made slower by lowering the voltage of drive pulses for precision positioning, but when the voltage of drive pulses lowers below the critical value, no displacement is caused.
More specifically, FIG. 5 is a view showing relationship between drive pulse voltage applied on the piezoelectric transducer and displacement; FIG. 5(a) shows a pulse voltage applied in a predetermined time, and FIG. 5(b) shows displacement generated in a predetermined time. As shown in FIGS. 5(a) and (b), when the drive pulse voltage exceeds the critical value as shown by a line Va, such displacement as indicated by a line Da adapted to the drive pulse voltage is caused, but when the drive pulse voltage is the critical value or less as shown by a line Vb, no displacement adapted to the drive pulse voltage is caused as shown by a line Db.